With significant advances in inkjet technology in recent years, it is becoming possible for inkjet image forming devices to produce large color prints of high quality at high speeds. Inkjet image forming devices are widely used particularly in sign and display applications, and are applicable to, for example, prints on POP (Point Of Purchase) posters, wall posters, outdoor advertisements, billboards, etc. Inkjet image forming devices are capable of producing prints by forming a number of ink dots, approximately several tens μm in diameter, on a print medium by applying droplets of different color inks, e.g., C, M, Y, K inks, to the print medium.
It is known in the art, according to human visual response characteristics, that human color vision is most sensitive to shades of gray. More specifically, if dots printed in different colors are of the same size, then dots in a K ink can be perceived visually better than the dots in other C, M, Y inks. Therefore, an observer of an image produced using an increased amount of K ink sees the image as grainy in its entirety (the granularity level is high) although the observer fails to identify individual dots.
If dots printed in C, M, Y inks are present closely together to form gray areas (composite black areas), then such dots provide a smoothing effect due to being superposed on each other, thereby lowering the image granularity level. However, since three inks are used rather than one ink, the total amount of inks used increases, thereby resulting in a high running cost (hereinafter referred to simply as “cost”).
According to inkjet technology, therefore, there is a trade-off between granularity level and cost, and it is important to design images so that granularity level and cost will be well-balanced. Various color conversion techniques have been proposed in the art for appropriately controlling the total amount of inks used by finely adjusting a dot recording ratio while keeping the dots substantially isochromatic.
Japanese Laid-Open Patent Publication No. 2000-078418 discloses a method of and an apparatus for predicting an image equality prediction value relative to the quality of a printed image based on a certain detected color signal, and determining the amount of a K toner to be used based on the image equality prediction value.
Japanese Laid-Open Patent Publication No. 2009-241609 discloses an apparatus for and a method of judging whether or not chromatic or achromatic dots are to be formed based on an input signal. The publication states that a pale K ink is used instead of a dark K ink in a color area where dot shapes are visible, thereby increasing the granularity level.